Country diary: Spotted regal sawflies make moves to repel me

Throughout the summer, two potted hazels have stood proudly in front of our bay window. Drawing back the curtains one morning, I immediately noticed that something didn’t look quite right – on some branches all of the leaf blades had been chewed down to their veins and midribs. When I went outside to investigate, I discovered hundreds of spotted regal sawfly (Nematus septentrionalis) larvae clinging to the foliage. At first glance, the 10mm-long green and black-spotted creatures could easily be misidentified as butterfly or moth caterpillars – but they had a clear distinguishing feature. In addition to their three pairs of forelegs, they sported six pairs of bright yellow prolegs, while Lepidoptera larvae have a maximum of five pairs.

While a few smaller individuals hid away on the underside of the leaves, most of the larvae were gripping the leaf edges and steadily munching their way inwards. They swayed hypnotically as they fed, a defence mechanism intended to confuse potential predators. When I extended a finger towards them, they reared up in unison, curling their bodies into an S shape in an attempt to repel me.

Sawflies are not flies. They belong to the order Hymenoptera, which includes wasps, bees and ants. In German, they’re known as Blattwespes, a more apt name meaning leaf wasp. Part of the sub-order Symphyta, they are the most primitive insects within the order and are believed to be closest to the ancestral form that all Hymenopterans evolved from, with the oldest sawfly fossils dating from the middle to late Triassic.

There are about 500 species found in Britain. Many are specialists, feeding on a single or limited number of plant species. Despite the spotted regal sawfly’s reputed preference for alder and silver birch, my two mature weeping birch trees had been spared, but the leaves of a small pussy willow had also been stripped to their skeletons.

From a gardener’s perspective, it seems difficult to comprehend the purpose of these destructive gluttons. But adult sawflies are pollinators of crops and flowering plants and, though it looks unsightly, the damage their progeny cause is mainly cosmetic and doesn’t have any adverse effects on the plant’s long‑term health.

• Country diary is on Twitter/X at @gdncountrydiary

• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 20% discount